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Exposure to phenolic compounds is associated with neuro-, myelo-, immue , geno-, and dermotoxicity in skin, phenols cause rash and inflammation, cotact and irritant dermatitis, leucoderma and cancer promotion. The biochemical mechanisms responsible for cytotoxicity of phenolic compounds are not well understood. We hypothesized that the cytotoxic effects of phenolic compounds are due, at least in part to the generation of phenoxyl radicals via the enzymatic one-electron oxidation which alters the intracellular pool of GSH and protein sulfhydryls. To test this hypothesis we measured cell viability intracellular levels of GSH and protein SH-groups, as well as lipid peroxidation in NHEKs exposed to twelve different phenolic compounds. We found that an 18 h incubation of cultured NHEKs in the presence of 100-500 um 1,4-benzenediol (hydroquinone), o-hydroxybiphenyl (2-phenylphenol), bis(4-glycidyloxyphenyl)-methane, bis (4-hydroxyphenyl)-dimethylane (bisphenol A), 4-tert-butylcatechol, 1,2-benzenediol (catechol), iso-eugeinol (2-methoxy-4-propenylphenol), eugenol (4-allyl-2-methoxyphenol) or phenol caused pronounced cytotoxicity and significant depletion of GSH. In additition, concentration-dependent cell detachment, loss of cell-cell contacts, and disruption of the cellular membrane were observed. Importantly, the phenolic compounds that did not decrease thiol levels, such as 3-n-pentadecylphenol 4-tert-butylphenol, or bis (4-hydroxyphenyl) methane, did not induce cytoxicity after a 18 h treatment. Incubation of ascorbate-preloaded keratinocytes with phenols produced an EPR-detectable signal of ascorbate radicals indicating that redox-cycling of the one-electron oxidation product phenol, its phenoxyl radical, is likely involved in these oxidative effects. While phenolics are known to act as radical scavengers, their enzymatically formed metabolites, phenoxyl radicals, can react with vital thiol reductants and cause cytotoxic effects.